If you really want to sleep with your arms outside of the bag, I have no suggestions for how to do that and warm in winter if you are tenting.

The coldest temperature that I have ever seen on a thermometer was on the Canadian border while winter camping a few decades back. The thermometer in my hand read 36 below (F). We were camping under a tarp because a tarp does not hold in as much moisture and you do not want moisture to accumulate inside of a down bag.

I suggest you first learn to sleep with your arms in the bag and using the hood on the bag. Once you accomplish that, you simply need a bag and pad that is good for the temp. The tent is not the right way to add warmth.

On a different winter camping trip on the Michigan Upper Penninsula when there was four feet of powdery snow, I observed that you could tell who had the warmest pads because the depression in the snow the next morning was deepest for those that melted a few fractions of an inch deeper into the snow from their body warmth. With down bags, all of your insulation under you is in the pad, the down does not help because it compresses too easily.

I think that the VBL is most valuable in keeping your body moisture from getting into the down, not for warmth but it probably does add a few degrees. It is amazing how much moisture gets into the down in a few days of cold weather camping, even if you use a tarp that does not hold in the moisture like a tent does. When Stieger and Schurke (spell?) and others dog sledded to the North Pole, at one point their bags weighed several tens of pounds, most of which was accumulated moisture because they did not use a VBL.

As VT_speed_TR mentioned, wear a hat. I have a thin one with no tassel that I pack with the bag, thinner than the one I wear during the day because I do not want it falling off during the night.